Under normal flying conditions of multi engine aircraft, the air streams passing over the surface of the aircraft are substantially symmetrical, the controls are in the neutral position, the engines are producing substantially equal thrust, and the fuselage of the aircraft is substantially horizontally oriented.
In response to various external and internal conditions, these air streams sometimes become asymmetrical and cause asymmetrical aerodynamic forces to be subjected on the aircraft. An example of causes for such asymmetrical forces to be subjected on the aircraft is different engine thrusts being produced.
The design of multi engine aircraft must account for the possibility of an engine failure at low speed. The unbalance of thrust from a condition of unsymmetrical power produces a yawing movement dependent upon the thrust unbalance and the lever arm of the force. The deflection of the rudder will create a side force on the tail and contribute a yawing moment to balance the yawing moment due to the unbalance of thrust. Since the yawing moment coefficient from the unbalance of thrust will be greatest at low speed, the critical requirement will be at a low speed with the one engine out and the remaining engine at maximum or near maximum power; i.e. during take off and climb conditions.
Due to the side force on the vertical tail, which increases drag, a slight bank is necessary to prevent turning flight at zero sideslip. The inoperative engine will be raised and the inclined wing lift will provide a component of force to balance the side force on the tail. In each of the critical conditions or required directional control, high directional stability is desirable as it will reduce the displacement of the aircraft from any disturbing influence. Of course, directional control must be sufficient to attain zero silidslip.
The static lateral stability of an aircraft involves consideration of rolling moments due to sideslip. The axis system of an aircraft defines a positive rolling "L" a moment about the longitudinal axis which tends to rotate the right wing down. As in other aerodynamic considerations, it is convenient to consider rolling moments in the coefficient form so that lateral stability can be evaluated independent of weight, altitude, speed, etc. The rolling moment "L" in the coefficient form is as follows: EQU L=CqSb or C=L/qSb
where
L=rolling moment, ft/lbs. positive to the right PA1 C=rolling moment coefficient, positive to the right PA1 q=dynamic pressure, psi. PA1 S=wing area, sq. ft. PA1 b=wingspan, ft. PA1 1. Rolling moment due to sideslip; PA1 2. yawing moment due to rolling velocity; PA1 3. yawing moment due to sideslip; PA1 4. rolling moment due to yawing velocity . . . a cross effect similar to (2) above. (If the aircraft has a yawing moment to the right, the left wing will move forward faster and momentarily develop more lift than the right and cause a rolling moment to the right.) PA1 5. Aerodynamic side force due to sideslip; PA1 6. rolling moment due to rolling velocity or damping in roll; PA1 7. yawing moment due to yawing velocity or damping in yaw; and PA1 8. the moment of inertia of the aircraft about the roll and yaw axis. PA1 p-rate of roll, radians per second PA1 b=wing span, ft. PA1 V=aircraft velocity, ft./sec. PA1 and one radian=57.3 degrees PA1 D1=induced drag corresponding to original lift L1 PA1 D2=induced drag corresponding to new lift L2
The angle of sideslip is the angle between the airplane center line and the relative wind direction.
When the aircraft in free flight is placed in a sideslip, the lateral and directional response will be coupled; i.e. simultaneously the aircraft produces rolling moment due to sideslip and yawing moment due to sideslip. Thus, the lateral dynamic motion of the airplane in free flight must consider the coupling or interaction of the lateral and directional effects.
The principal effect to which determine the lateral dynamic characteristics of an aircraft are as follows:
The complex interaction of these effects produce three possible types of motion of the aircraft: (a) a directional divergence; (b) a spiral divergence, and an oscillatory mode termed Dutch roll.
Directional divergence is a condition which cannot be tolerated. If the reaction to a small initial sideslip is such as to create moments which tend to increase the sideslip, directional divergence will exist. The sideslip will increase until the airplane is broadside to the wind or structural failure occurs. Spiral divergence will exist when the static directional stability is very large when compared with the dihedral effect. Dutch roll is a coupled lateral-directional oscillation which is usually dynamically stable but is objectionable because of the oscillatory nature. The damping of this oscillatory mode may be weak or strong, depending on the properties of the aircraft on a disturbance from equilibrium and is a combined rolling-yawing oscillation in which the rolling motion is phased to precede the yawing motion. Such motion is most undesirable because of the great havoc it creates in aircraft controllability.
The lateral control of an aircraft is accomplished by producing differential lift on the wings. The rolling moment created by the differential lift can be used to accelerate the airplane to some rolling motion or control the airplane in a sideslip by opposing dihedral effect. The differential lift for control in roll is usually maintained by some type of ailerons.
When an aircraft is given a rolling motion in flight, the wing tips move in a helical path through the air. The resulting angle between the flight path vector and the resulting path of the tip is the helix angle of roll. EQU Roll helix angle=pb/2V (radians)
Where:
Generally, the maximum values of roll helix angle obtained by control in roll are approximately 0.1 to 0.07. The helix angle of roll is actually a common denominator of roll performance.
The preceding teachings summarize the complexities at play in retaining aircraft control upon engine failure. When a multi engine aircraft, twins in particular, losses an engine during the early phases of take-off, the procedure for the pilot is to first avoid the rolling motion into the dead wing engine which could result in a spiral or inverted flight. The pilot lowers the "active" wing by applying ailerons. The "active" wing is the wing whose engine is still under power. The left and right wing ailerons are coupled and therefore two control surfaces are deployed and thereby produce double drag. As known, induced drag varies as the square of the lift, as follows: EQU D1/D2=(L2/L1)(L2/L1)
Where:
When the ailerons are activated, the airplane yaws toward the lowered wing thereby affecting direction stability and sideslip. To counteract this effect, the pilot applies opposite rudder to restore direction control and zero sideslip. Therefore, the rudder further increase drag.
Assuming a fast response by the pilot (3.5 seconds is required by the FAA) the airplane has now only 20% thrust available from the original 100% prior to engine shutdown. This is due to the drag of the dead engine and the control surfaces deployed to avoid loss of heading, roll-over and sideslip. Therefore the aircraft is now at the limit of its safety envelope.
By the use of the instant invention, one of (a) differential longitudinal forces delivered to the aircraft by the engines or (b) undesirable rotation of the aircraft about its longitudinal axis "c" is detected, a single control element or surface with a minimum drag or a thruster is activated which will achieve the lowering of the "active" wing, maintain directional control without the use of the rudder and retain zero sideslip.
Upon reestablishing directional control, the airplane is kept, via aerodynamic feedback loop in a control flight attitude well within its safety envelope.
When incidents occur which cause these undesirable forces, such forces must be quickly corrected or the aircraft may become uncontrollable. Various technologies have been developed to help control such problems. However, heretofore utilized technologies are often excessively and undesirably complicated, have reliability problems, are costly, not automatic nor independent of other controlling surfaces, and have an undesirably slow response time while failing to address the problem of excessive drag, thereby putting the aircraft in a marginal safety situation.
The present invention is directed to overcome one or more of the heretofore problems, as set forth above.